1. Field of the Invention
The present invention generally relates to a suspension control system, and more particularly to a suspension control system for controlling a damping coefficient of a variable damping coefficient type shock absorber on the basis of a running condition of a vehicle.
2. Description of the Related Art
There is previously known a suspension control system in which the damping coefficient of a shock absorber is controlled on the basis of the rate of change of the damping force. When the change rate of the damping force exceeds a predetermined value, that is, when the change rate changes abruptly due to a rough road surface or braking, the damping coefficient force with respect to a movement of the shock absorber is rapidly changed to a small level, so that the control response characteristic of the shock absorber is improved.
There is also known a suspension control system in which an adjustment reference value provided for the damping force change rate used for changing the setting of the damping coefficient is altered on the basis of a driving condition, such as a vehicle speed, so that ride comfort is improved (see Japanese Laid-Open Patent Application No. 64-67407).
The conventional suspension control system proposed in the above-mentioned Japanese Application has an advantage in that the damping coefficent is rapidly changed in accordance with the road surface condition so that good ride comfort is obtained. However, there is room for improvement in ride comfort in a case where the vehicle is continuously traveling on a rough road surface and thus the damping force change rate frequently changes around the adjustment reference value. In such a case, the setting of the damping coefficient is altered each time the damping force change rate exceeds the adjustment reference value. However, such a setting control of the damping coefficient does not provide good riding comfort, driving controllability and stability. In order to eliminate this problem, conventionally, the setting of the damping coefficient is altered when the damping force change rate exceeds the adjustment reference value and then maintained for a predetermined fixed time period. However, it is very difficult to very reliably control the setting of the damping coefficient in response to the road surface condition by simply maintaining the altered setting of the damping coefficient for the fixed period. For example, the vehicle is traveling on a rough road surface, the damping force change rate changes greatly. Thus, if the fixed period is short, the setting of the damping coefficient will be frequently altered, which deteriorates durability or lifetime of the shock absorber. On the other hand, if the fixed period is long, a problem will occur in which the setting of the damping coefficient unnecessarily maintained in the soft state, for example, after the vehicle rides across a minor stepped road surface portion. In such a case, the road holding ability of the vehicle will deteriorate and driving feeling will also deteriorate.